The Trypanosomatid cytoskeleton consists of a corset of regularly spaced microtubules linked to each other and to the closely overlying plasma membrane. We are interested in the composition of the cross-links and in how microtubules are bound to membranes. We have initially focussed on 3 proteins (designated, relative to their subunits Mr, COP-33, COP-40 and COP-61) because of their prominence in the isolated cytoskeleton, and because 2 of them also cross-linked microtubules to each other in vitro. We have purified each and noted the following properties, listed in the above protein sequence. Detergent needed for solution: yes, no, yes. Oligomeric state in solution: tetramer, tetramer, dimer. Crosslink microtubules in vitro (periodicity/length in nm): 8/12, 8/18, none. Binding to microtubules: maximum mol/mol - 0.3, 0.6, 0.3; apparent Kd (x10-7 M)-3.2, 3.7, 1.3; positive cooperativity - yes, yes, yes. Rabbit antisera to each COP, adequate in titer and specificity, have been used together with gold-labeled secondary antibodies to begin to localize the antigens within these very small cells. COP- 41 antibody bound to glycosomes, using thin sections from etched soft resin, and we believe it is a glycolytic enzyme which binds to the corset apparatus only after cell disruption. With this type of cell preparation the COP-33 antigen has not yet been detected. Gold-labeling by anti COP-61 appears similar to that by antitubulin, infrequent but concentrated in the corset-plasma membrane region.